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JOURNAL of AES HOLGER LAURIDSEN.Pdf PAPERS The Reproduction of Sound in Auditory Perspective and a Compatible System of Stereophony* FR. HEEGAARD Statsradiofonien of Denmark A little more than a year ago we invited the late Holger Lauridsen to contribute to the E.B.U. Review an article describing his work on stereophonic sound reproduction and in particular the system that he had proposed for stereophonic broadcasting without most of the disadvantages of using separate transmitter chains for the left-ear and right- ear components. Many of our readers will recall Mr. Lauridsen's untimely death in December 1957, which put an end to his valuable contribution to the art and science of broadcasting. Recently, however, interest in the possibilities of stereophony, and particularly in stereophonic broadcasting, has become widespread, and Mr. Heegaard very kindly agreed to write the following article, which is based upon the work of Mr. Lauridsen. Editor LAURIDSEN'S M-S TECHNOLOGY by Blumlein insum-differencesignalformats, as found in the references. The experimentsof Holger Lauridsenat DanishState DUANEH. COOPER Radio, leading to demonstations in Copenhagen in Chaml_aign,IL 61820, USA 1954June, dealt with the perceptionof space, evoked in allocating two channels, a main channel (M) to direct sound and a second (S) to reverberant sound --to be reproduced with either headphones or * This paper is reprinted from E.B.U. Review, Part loudspeakemmand the qualities of the sound-field A_Technical, no. 52, pp. 2-6, 1958 December, illusion that depend on delay insertion and polarity published by the Technical Centre of the European inversion. Reproduction of spatial sound by M-S Broadcasting Union. loudspeaker array--a transverse dipole radiator (S) Reprinting here is intended to honor the seminal work in coincidence with a monopole radiator (M)--was of Holger Lauridsen in exploring spatial perspective shown to be an alternative to reproduction by spaced in sound reproduction and in calling attention to sum channels, left as M + S and right as M - S. In the -difference, or M-S, matrixing in his own research reproduction of images, by contrast, stereophonic and in the (at the time widely neglected) earlier work qualities and lateral spread were observed in the re- of Alan Blumlein. The term "MS" originates with productions of sound from the M-S microphone Lauridsen, surely one of the second-wave, if Blumlein array--a cardioid directed inmidplane at center stage is one of the first, stereo pioneers. Lauridsen's original (M) and a dipole rotated 90° for side (S) pickup.-The M-S microphone, as shown in Fig. 4 of this paper, mnemonics mid-side or mono-stereo are suggested, remains on exhibit at the Danish Radio Museum in with left and right (or twin-channel A, B) signals con- Copenhagen. Lauridsen's own writings are in Danish structed by sum and difference. Compatible M-S and German, for which translations to English, unfor- stereo relies on forming M as A + B and S as A - tunately, are not readily available. We are indebted to B in applications for which M and S are suited as the Danish Radio Corporation for their cooperation, transmission, FM broadcast, or recording channels, particularly to Dan Popescu for his assistance and to Problems with phase incoherence for spaced micro- Peter Hansen, Direction of Engineering (Retired), who phones are noted, along with mention of prior work prepared the biographies published with this paper. 802 J. AudioEng.Soc.,Vol.40,No.10.1992October PAPERS AUDITORY PERSPECTIVE AND COMPATIBLE STEREOPHONY INTRODUCTION impression of an added dimension to the imaginary room in which the music seems to take place. The There seems to be little doubt that any major im- resemblance of this arrangement to that of Vermeulen provement in the quality of broadcast sound reproduc- and Kleis at Eindhoven will be recognized [5], [6]. tion must be towards the creation around the listener Feedback may be applied to the delaying machine of a sound field having a spatial distribution approxi- in order to simulate secondary reflections, thus ap- mating much more closely to that in the broadcasting proximating more closely to the reverberation process studio than is the case with ordinary "single-channel" in a real room. Although this does add some naturalness broadcasting. It may even be said that the improvements to the effect, it is surprising to note the relatively small brought about by the introduction of VHF broadcasting, importance of the amount of feedback used. It seems leading to a better reproduction of the higher audio to be the first few reflections that create the impression frequencies, have not always led to happy results, be- of a large room. cause the radiation of strong components at those fre- The experiment is rather instructive, because it dem- quencies from a concentrated source has often been onstrates the fact that naturalness of reproduction may found to Cause a certain irritation, especially when be greatly increased without establishing any possibility broadcasting from extensive sources, such as an or- of right and left localisation. When listening to a re- chestra. It is true that the introduction of spherical cording of organ music it is quite easy to imagine that loudspeakers may to some extent alleviate this effect, one is present in a large church. but even this does not cure the trouble completely, It is perhaps not very surprising that the arrangement because the loudspeaker remains essentially a point described creates such an impression, seeing that the · source which is easily located by the listener, sound from the loudspeakers approximates to what Certain experiments which were carried out in the would be obtained in a large room. It is rather less laboratories of the Danish State Radio and which shed easy to imagine exactly by what mechanism of hearing some light on the problems of spatial sound reproduction the psychological effect is obtained. Since, however, will be described in this article. These experiments led the effect must be obtained solely as a result of the to a compatible system of stereophony which was dcm- sound pressure at each ear as a function of time, one onstrated for the first time at a meeting in "Nordisk might well ask whether the use of spaced loudspeakers Akustisk Selskab" in Copenhagen on 6th June, 1954 is really necessary. Could not similar spatial effects be [1], [2]. obtainedwhen listening with a pair of headphones? The answer was found to be yes, and the method of SOME EXPERIMENTS PRODUCING A achieving these effects is both simple and surprising. SUBJECTIVE EFFECT OF SPACE Consider Fig. 1. We have a pair of headphones to which is delivered a signal from a musical recording. The first experiment is very simple: A loudspeaker The headphones are connected so that the diaphragms is placed in front of an observer and another behind move in phase; by this is meant that, at any instant, his back. Musical signals are fed to the loudspeakers, they both move towards the head or they both move The effect of the back loudspeaker is to some extent away from it. A delayed signal from the same source to lessen the localisation of the front loudspeaker as a may also be fed to the headphones through a separate sound source. The distances between the observer and circuit, and in the leads to one of the phones in the the two loudspeakers are rather critical and the effect delayed circuit is connected a reversing switch. The is not very marked although a certain sense of roominess delayed signal may thus be led to the headphones in is achieved, phase or in antiphaseat will. The delay is again of the If, however, the sound from the back loudspeaker order of 50-100 milliseconds. is delayed by means of a tape recorder by some 50- We start the experiment by listening to the primary 100 milliseconds, the effect is completely different. The observer now gets the impression of being present in a large room in which music is played in front of him. The loudspeaker distances are no longer critical, recordmgv.,,,,. because localisation of the back loudspeaker is sup- pressed by the Haas effect. This is a modern version _ [-5-1 '1 1926, which consisted of a gramophone with two sound- I boxes placed one behind the other on the record and eachleadingto itsownhorn. [-5-3 of Thethe "Ultraphon"arrangement demonstratedmay be improvedby Kiichenmeisterby the additionin [ _ ___U.,jp_...... the observer. The lateral loudspeakers are fed in parallel of a loudspeaker to the left and another to the right of _ ] [_Z3 0'_ fSo° with delays of some 50-100 milliseconds, the delay {-5'-3 of the back loudspeaker being then 100-200 milli- seconds. This arrangement gives the observer the Fig. 1. Circuit for reproducing sound with an effect of space. J. Audio Eng. Soc., Vol. 40, No. 10, 1992 October 803 HEEGAARD PAPERS signal only. The effect is the usual one when listening shot seems to start in the back of the head and fly away. with headphones. The sound seems to come from no- Another loudspeaker arrangement has also been used where in particular or seems perhaps to be localised in for these experiments. This is shown in Fig. 3. One the head itself. We add some delayed sound, first with loudspeaker is placed in the usual fashion in front of the headphones connected in phase. The effect is that the observer and a second, which we shall call the of added reverberation and, due to the rather long delay, transverse loudspeaker, is placed behind the first and we soon arrive at a point in which the clarity of repro- with its edge towards the observer.
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